Sensor Uses Qdots to Detect DNA

Photonics.comDec 2005
BALTIMORE, Md., Dec. 6 -- Using tiny semiconductor crystals, biological probes and a laser, Johns Hopkins University engineers have developed a way to find specific sequences of DNA by making them light up beneath a microscope.

The researchers, who say the technique will have important uses in medical research, demonstrated its potential in their lab by detecting a sample of DNA containing a mutation linked to ovarian cancer.

The technique involves an unusual blend of organic and inorganic components.

Jeff Tza-Huei Wang, supervisor of the research team and senior author of a paper on the new DNA nanosensor (Nature Materials), said, "This new technique is ultrasensitive, quick and relatively simple. It can be used to look for a particular part of a DNA sequence, as well as for genetic defects and mutations. This method may help us identify people at risk of developing cancer, so that treatment can begin at a very early stage."

Quantum dots, or qdots, are crystals of semiconductor material that are in the range of a few nms across. They are traditionally used in electronic circuitry. In recent years, however, scientists have begun to explore their use in biological projects.

Wang led his team in exploiting an important property of quantum dots: They can easily transfer energy. When a laser shines on a qdot, it can pass the energy on to a nearby molecule, which in turn emits a fluorescent glow visible under a microscope.

But qdots alone cannot find and identify DNA strands. For that, the researchers used two biological probes made of synthetic DNA. Each of these probes is a complement to the DNA sequence the researchers are searching for, so the probes seek out and bind to the target DNA.

To create their nanosensor, the researchers mixed the two DNA probes with a quantum dot in a lab dish containing the DNA they were trying to detect. Then nature took its course. First, the two DNA probes linked up to the target DNA strand, holding it in a sandwich-like embrace. Then the biotin on one of the probes caused the DNA "sandwich" to stick to the surface of the qdot. Each qdot can connect to up to about 60 DNA sequences, making the combined glow even brighter and easier to see.

To test the new technique, Wang's team obtained DNA samples from patients with ovarian cancer and detected DNA sequences containing a critical mutation. The Johns Hopkins University has filed for a provisional patent covering the DNA nanosensor technology. Funding for the research was provided by the National Science Foundation and the Whitaker Foundation. For more information, visit: www.jhu.edu

Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.